1. Field of the Invention
The invention relates to a communications system used for wireless communications and, more specifically, to a technique for realizing high-performance wireless communications using a plurality of receivers or transmitters.
2. Description of the Related Art
With the recent increase in demand for wireless communications represented by computer communications, there is a growing need for technology that can transmit broadband digital signals and analog signals while maintaining high signal quality. As a result, some frequency bands, particularly those higher than the SHF band, have come into use due to their wide bandwidth and the shortage of available frequencies in the microwave band. These bands are now being used in, for example, millimeter-wave-based video image multiplex transmission systems, wireless LAN, wireless home-link, and wireless road-vehicle (or vehicle-vehicle) communications systems.
In many wireless communications systems using high radio frequencies, the transmitter generates modulated signals in an intermediate frequency band, multiplexes two or more channels, and then converts (upconverts) the modulated signals into those of a predetermined radio frequency band using a local oscillation source. The receiver, in turn, converts (downconverts) the received signals of a radio frequency band into those of the intermediate frequency band using a local oscillation source, and then extracts the channels and demodulates the modulated signals.
During this process of signal exchange, the local oscillation sources used in the transmitter and the receiver must be synchronized with each other. If there are time-varying fluctuations in frequency and signal phase between the employed oscillation sources, the fluctuations overlap the modulated signals that have been downconverted, and degrade the quality of demodulated signals.
Thus, both the transmitter and receiver require a local oscillation source with high frequency-stability. It is, however, technically difficult to realize a local oscillator that provides high frequency-stability even at high frequencies. Even if such an oscillator becomes available, its manufacturing cost will be very high, and the wireless communications system employing such an oscillator will be expensive as a result.
Furthermore, because a wireless communications system using a high-frequency band such as the millimeter-wave band must receive direct signals, the path of radio waves is often blocked by obstacles such as humans, making it difficult to carry out stable wireless communication.
To solve such problems, the applicant of this invention has disclosed a self-heterodyne-type wireless communications system in Japanese Patent Kokai publication No. 2001-53640. In this disclosure, the signals for transmission are converted by a local oscillation signal from those of an intermediate frequency band into those of a radio frequency band, and the local oscillation signal and modulated radio-frequency-band signals are transmitted together wirelessly. Upon signal reception, the oscillation signal and modulated signals are mixed to restore the signals of the intermediate frequency band.
Such a self-heterodyne-type system having a receiver with a simple structure can transmit signals of high quality and reduce the cost of wireless communications systems.
To solve the signal-blocking problem, it is effective to adopt a diversity technique employing a plurality of independent transmitters or receivers. However, the prior-art structure involves some problems when it is used in the millimeter-wave band and its vicinity.
Specifically, according to a prior-art technique known as switching diversity, in which signals are transmitted or received by switching transmitters or receivers, switching noise is generated by the switching operation during the transmission or the reception of signals such as video images. Furthermore, if there is a large frequency offset upon switching, it is difficult for the demodulator circuit to quickly respond to such an offset, and thus it is also difficult to maintain stable wireless communication.
In addition, according to a technique known as “combining diversity,” in which a plurality of independent signals for transmission/reception are combined in-phase, it is difficult to prevent the occurrence of frequency differences between the independent oscillators, and it is also difficult to synchronize signal phases during signal combining due to independent phase fluctuations (phase noise).